The present invention relates to the field of documents scanners and, more particularly but not exclusively to document scanners used with notebook and laptop computers and personal digital assistants (PDA's). 2. Description of the Prior Art
Historically, document scanners have been large, flatbed, desktop devices that occupy a footprint on a worksurface having an area which is larger than the largest document to be scanned, e.g. larger than 81/2.times.11 inches. Smaller, sheet-fed document scanners have been developed in which during scanning a document travels along a U-shaped or curved path through the document scanner. Also document scanners have been developed in which a handheld wand is drawn across the surface of a document. All three of the preceding types of document scanners, i.e. flatbed scanners, curved path scanners, and wand scanners, have been connected by a signal and power cable usually to a desktop computer. Because a desktop working environment imposes essentially no limitation on how much electrical power the document scanner consumes, these types of document scanners generally do not use battery power.
Presently, PDA usage is expanding rapidly with various companies offering competing products. These PDAs are being used widely both for displaying information, and for capturing information in real-time. However, generally real-time capture of information using a PDA requires manual entry of data either using a miniaturized keyboard, or by cursive writing with a stylus on the PDA. Accordingly, incorporating a very small document scanner into a PDA, or attaching a document scanner to a PDA, would significantly enhance their usefulness for data entry. For example, small documents, such a business cards, could be scanned directly into the PDA. Similarly, rather than writing onto the PDA with a stylus, data being entered into the PDA could be simply printed or written onto a sheet of paper that is then scanned. The scanned image thus obtained may then be converted to digital data within the PDA using optical character recognition ("OCR") or intelligent character recognition ("OCR") techniques.
Present commercially available handheld document scanners generally use an optical diode array or a charge coupled device ("CCD") for converting an optical image into an electronic signal. A roller, gear and encoder mechanism is also included in the handheld document scanner to synchronize image data acquisition by the CCD with mechanical movement of the scanner across the document. Thus, if the document scanner provides 300 line-per-inch ("LPI") resolution, the encoder must generate 300 line-scanning pulses to capture 300 lines of image data for each inch of scanner movement. If the roller has a 1 inch circumference, then the gear train and encoder must be comparatively large and elaborate to generate the required 300 line-scanning pulses per roller revolution.
Another characteristic of such commercially available handheld document scanners is that they usually indicate on a display screen of the computer if scanner motion is too fast for image capture. The requirement that a user must watch the screen of the computer while drawing the handheld document scanner across a document tends to inhibit, to some degree, smooth scanner motion.
Also, many commercially available handheld document scanners lack an automatic exposure control. Using a CCD sensor, there exists an optimum scanning speed to ensure a proper exposure. If the scanner moves too slowly, the image will be over-exposed. If the scanner moves too fast, the image will be under-exposed. Thus, a document may have to be scanned several times using such a document scanner before obtaining a useful exposure. Repetitively scanning a document to obtain an acceptable image is very time consuming and frustrating.
One can readily obtain an appreciation for the technological challenge presented in capturing high quality images using a handheld scanner by considering imaging a three inch wide document at 400 dots-per-inch ("DPI"). If such a document is to be scanned at 3,000 lines per second using an oscillating flying spot scanner with all lines being scanned in the same direction, then each line must be scanned in approximately 166 microseconds. Since there are 1,200 dots in each scan line, the data for each dot, generally called a picture element ("pixel"), must be acquired in 138 nanoseconds, i.e. at approximately a 7.2 MHz pixel data rate. capturing an image at this resolution and speed provides a high quality scanned image that is virtually free of motion fuzziness. Thus, if the motion of a handheld document scanner were irregular, the scanned image may be distorted by jerky scanner motion, but the image is not blurred. Such an operating capability is particularly important in scanning small type fonts, such as those used on business cards, so the image is not to be smeared.
In addition to severe technological constraints of small physical size, extremely high optical performance, and low electrical power consumption, there also a severe price constraint for document scanners to be used with PDAs. Since PDA's have now become a consumer item, prospective purchasers resist paying significantly more to add document scanning capability to their PDA. Consequently, a commercially practical document scanner used with a PDA must be priced commensurate with the PDA, while producing the high quality and high resolution images needed to capture small type fonts printed on business cards or other documents. In addition to being useful with PDAS, any document scanner that adequately solves the technological and commercial constraints imposed by PDAs would also be useful with notebook and laptop computers.